Typical examples of resins which are used in metallic foil-covered laminates are phenol resins, epoxy resins, unsaturated polyester resins and so forth, as described, for example, in U.S. Pat. Nos. 3,902,951, 3,897,588 and 4,440,833. They are used in the form of composite materials in combination with various substrates.
In general, these laminates have been produced by use of phenol resins with paper as a substrate or epoxy resins with a glass cloth as a substrate, under pressure, by heat curing In these cases, however, material costs and equipment costs are high, and further the processes are complicated. Furthermore, in accordance with the above press molding method, surface roughness of a sheet or plate to press the laminates is transferred to the laminate surface and, therefore, it is necessary to apply brushing onto the laminate surface. In order to solve the above problems, a method has been recently developed in which a paper substrate is impregnated with liquid unsaturated polyester resins and continuously molded and cured by heating. However, copper-covered laminates using unsaturated polyester resins are inherently poor in heat resistance, and thus when the laminates are dipped in a solder bath for a long time, they deform to such an extent that they are difficult to be handled. This disadvantage reduces the reliability concerning the durability of the products and inhibits the widespread use of the laminates. In order to overcome the above problem, Japanese Patent Application (OPI) No. 46970/80 (the term "OPI" as used herein means a "published unexamined patent application") discloses that combinations of monofunctional hydrocarbon monomers and polyfunctional hydrocarbon monomers are used as crosslinkable monomer for unsaturated polyesters. However the problem has not yet been sufficiently overcome.
Resins known as vinyl ester resins are not widely used in laminates although they are excellent in physical properties of the cured products thereof.
Conventional vinyl ester resins have a molecular weight of several hundreds to 2,000. In impregnation of paper or glass substrate and heat curing, the resins flow seriously from the substrates, and furthermore in addition of other monomers, a considerable reduction in viscosity occurs.
In order to solve the above problems, a method of compounding silica for the purpose of imparting thixoviscosity has been employed. Imparting thixoviscosity, however, undesirably lowers a rate of impregnation to substrates such as paper or glass.
Furthermore the vinyl ester resins are made mainly of epoxy acrylate and, therefore, they are relatively expensive and have not been widely used.